T9B

Feed lines, SWR and connectors

Why coax loses more at higher frequencies, what SWR and a tuner really do, and which connector fits where.

12 pool questions1 on the exam

Syllabus: Feed lines: types, attenuation vs frequency, selecting; SWR concepts; Antenna tuners (couplers); RF Connectors: selecting, weather protection

Coax is the easy feed line, usually 50 Ω

A feed line carries RF between radio and antenna. Coax has a center wire inside a shield, so you can run it near metal and walls with few special precautions. Amateur coax is almost always 50 ohms.

One conductor inside another, sharing a center line: that's what 'co-axial' means.

RememberCoax: easy to install. 50 Ω.

Loss grows with frequency; bigger cable loses less

Every cable wastes some power as heat, and more of it as frequency rises. Thick RG-213 loses less than thin RG-58 at the same frequency. Air-insulated hardline loses the least.

Schematic: no scale on the loss axis. The order and the upward slope are what matter.

Frequency30.9 MHz

Other sources of loss: water in connectors, high SWR and extra connectors in the line. All of the above.

RememberHigher frequency, more loss. Thicker or air-insulated, less loss.

SWR measures how well the load matches the line

Standing wave ratio (SWR) rates how well the antenna matches the line: 1:1 is perfect. A mismatch bounces power back down the line.

SWR rates the match between the line and its load. 1:1 is perfect. The worse the match, the more is reflected.

Antenna impedance50 Ω

SWR on the line1:1

Power reflected0%

An antenna tuner (coupler) matches the antenna system's impedance to the transceiver's output impedance. A loose connection makes SWR jump around erratically.

RememberSWR = match quality. Tuner = impedance match. Erratic SWR = loose connector.

Pick the connector for the frequency, then tape it

Match the connector to the frequency. Weatherproof every outdoor connector with tape.

PL-259 is the common HF and VHF connector. Type N is the choice above 400 MHz. A BNC has a bayonet twist lock.

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Compendium articles on the ideas in this lesson, beyond what the exam asks.

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Practise the 12 pool questions for this group. Answers are shuffled, and every one comes with a one-line "why".

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